54 
A Convenient Form oj Potometer . 
The middle hole is destined to receive the stem of the shoot of 
which the rate of transpiration is to be measured, whilst the right- 
hand hole allows a thisrle funnel (f), provided with a stopcock, to 
pass through it. 
The chief advantage in the rubber cork lies in the ease with 
which the plant stem can be passed through it without injury, and 
gripped so as to be air tight. In order to do this, a blunt cork- 
borer smeared with oil or vaseline, and somewhat larger than the 
diameter of the hole is worked through it. The shoot is then 
passed into the tube of the cork-borer, and the latter withdrawn. 
The stretched rubber contracts upon the stem and if a shoot 
of appropriate thickness has been chosen, a perfectly air-tight 
enclosure is the results as soon as the cork is compressed by the 
neck of the bottle, It should be observed that only woody stems 
are suitable, as otherwise there is the risk of the tissues being too 
much compressed to allow of the free passage of the transpiration 
current. 
To start the apparatus, after the cork is provided with its 
proper complement of tubes, funnel and shoot, all that is required 
is to fill the bottle with water and then to press in the cork. The 
funnel (which of course must be higher than the horizontal limb of 
the tube t) is filled with water, and the tap opened so as to allow 
